1. Field of the Invention
This invention generally relates to electronic circuits, and more particularly, to an electronic circuit having a control circuit that controls a gain of an amplifier by use of an output from the amplifier.
2. Description of the Related Art
A description will now be given of, for example, a transimpedance amplifier (hereinafter, referred to as TIA) as an electronic circuit having an automatic gain control circuit (hereinafter, referred to as AGC circuit, gain control circuit) therein. The AGC circuit controls a gain of an amplifier by use of an output from the amplifier used as an amplifier for optical communication. A TIA is an amplifier circuit that converts current into voltage and amplifies the voltage, and is used for, for example, optical communication. At a light receiving side of the optical communication, a light receiving element such as a photo diode or the like converts the beam transmitted by means of the optical fiber into current. The TIA amplifies the current output from the light receiving element to output as a voltage signal. The input into the TIA is composed of input current of DC component and input signal of high-frequency component. There is an input current range that the TIA can amplify with high sensitivity. For this reason, if the magnitude of the input current is varied by a large amount, the TIA cannot amplify the input signal with high sensitivity. Therefore, in some cases, the TIA includes an AGC circuit. The AGC circuit controls the gain of the TIA according to the output voltage of the TIA. If the output voltage of the TIA is small, the AGC circuit does not control the gain. However, as the output voltage is increased, the gain of the TIA is decreased. It is therefore possible to produce an amplifier that operates with high sensitivity, from a small input current to a large one. Not only an optical signal of weak current but also that of a strong one can be amplified with high sensitivity, by amplifying the output current of the light receiving element by use of the TIA circuit that includes the AGC circuit therein. Japanese Patent Application Publication No. 2003-258580 discloses the TIA circuit that includes the AGC circuit therein.
It is to be noted that the output from the amplifier is inappropriately reflected on the gain control of the amplifier, in the amplifier that includes the AGC circuit therein. For example, if the signal to be input into the TIA that includes the AGC circuit therein is blocked off, the voltage output from the TIA will abnormally become high and a circuit at the subsequent stage will malfunction.